In many applications, there is a need to effectively and efficiently clean filters. In particular, there is a need to clean expensive filters, restoring them to a condition in which the filters can be reused. Additionally, many filters are difficult to clean using conventional means. A common method of cleaning filters uses forced air to blow off the retentate. There are a number of disadvantages to using forced air for filter cleaning.
First, forced air methods and systems generate airborne particles and dust that may pose health and environmental hazards. Many forced air cleaning systems require large and expensive secondary dust collection and filtration systems.
Second, forced air is not always effective at removing retentate from filters. In particular, filters with small pores, channels, pleats, or the like; or filters in which the retentate has become sintered, stuck, or otherwise adhered to itself or the filter surface may not effectively be cleaned by the use of forced air.
Third, forced air filter cleaning methods are ineffective at removing retentate packed in areas of the filter that experience little-to-no airflow.
Finally, ineffective filter cleaning reduces the useful life of the filter, and may cause efficiency losses or damage to the systems in which the improperly cleaned filters are used.
While there are many types of filters, diesel particulate filters, in particular, are especially difficult to clean using forced air. Diesel particulate filters are used to reduce soot emissions from a wide range of sources, including internal combustion engines. Although there are many types of diesel particulate filters, the most common are ceramic honeycomb-type wall-flow filters. These filters may be cordierite, silicon carbide, mullite, aluminum titanate, or any other suitable material. The wall-flow filters generally contain thousands of small channels, the walls of which are composed of a porous material. The ends of the channels may be alternately blocked at opposite ends of the filter (checkerboard pattern). Particulate-laden exhaust that enters the open end of a filter inlet channel is forced to flow through the porous filter walls to the adjacent outlet channel. The soot particles in the exhaust are trapped in the wall pores or in a cake layer along the channel surface.
In addition to ceramic wall-flow filters, a number of other diesel particulate filters exist including metal foam, sintered metal, and fiber-based (ceramic, glass, paper, and others) filters. These filters may contain pleats, channels, or a porous matrix, among other configurations.
Diesel particulate filters are effective at trapping in excess of 99% of engine-out soot in some cases. The trapped soot is periodically or continuously removed from the filter by regeneration. The regeneration process oxidizes the soot, but leaves incombustible ash behind.
Over time, the incombustible ash builds up in the filter, either in a layer along the filter walls, or in a plug at the back of the filter channels. Ash generally consists of metallic components, originating in lubricant additives, such as Mg, Ca, and Zn forming various sulfates, phosphates, and oxides. Trace metals in fuels (Na, K, Ca, and other elements found in biofuels for example) and engine wear metals and corrosion particles may also contribute. In cases where fuel-borne additives are used for filter regeneration, such as Ce, Fe, Pt, and others, these elements may also contribute significantly to ash accumulation in diesel particulate filters.
The build-up of ash restricts exhaust flow, increases exhaust backpressure, and negatively impacts engine fuel consumption. The ash further reduces the filter's soot storage capacity. In extreme cases, filter plugging can cause excessive exhaust backpressure leading to failure of the engine, exhaust, or machine system. One means of mitigating the negative effects of ash build-up on the filter and system (engine, vehicle, etc.) performance is to clean the filter, i.e., to remove the ash.
Common systems and methods employed to clean diesel particulate filters include forward and reverse blowing, through the use of forced air. Additionally, the filter may be heated (generally up to 650 C) to oxidize and remove soot as well. While filter heating is effective at removing the soot, it is relatively ineffective at removing ash at these temperatures.
One reason that diesel particulate filters are difficult to clean is due to the manner in which the ash accumulates in the filter, as well as its composition. Initially, beginning with a clean filter, the ash accumulates in a thin layer along the filter walls. This thin layer or membrane may actually provide a reduction in filter pressure drop when the filter is also loaded with soot, as the ash layer may prevent soot from accumulating in the filter pores (depth filtration). Generally, the beneficial ash membrane is formed with less than 15 g/L specific ash loading, resulting in an ash layer that is on the order of 50 microns thick; however the pressure drop benefits may be realized with much thinner ash layers and lower specific ash loadings. Depending on the ash composition and properties, these values may differ, however.
As more ash accumulates in the filter, the pressure drop may increase significantly, such that any initial benefit is lost, and for a given level of soot load, the pressure drop across a particulate filter containing soot and ash is greater than the pressure drop across a filter containing the same amount of soot but no ash. Specific ash levels above 25-30 g/L may result in a significant increase in filter pressure drop, loss of soot storage capacity, as well as increase in regeneration frequency.
At these elevated ash levels, the majority of the ash (75% or more) may be packed toward the back of the filter, forming plugs in the filter channels. In some cases, for filters with 150,000 miles of on-road use, the ash may completely plug over 50% of the length of the DPF channels. The ash accumulated in the channel end-plugs may also pack, sinter, or fuse together or to the filter surface. As the ash accumulated in the plugs completely blocks the flow of exhaust through the ash-plugged region of the filter, it contributes significantly to filter pressure drop, generally more so than the porous ash accumulated in a thin layer along the filter walls.
Considering the ash distribution in the DPF, an ideal cleaning process is one that removes all of the ash from the plugs, but leaves a small thin layer along the channel walls. Removing the ash from the end plugs significantly increases the available area for soot deposition, and reduces pressure drop. Leaving a thin layer of ash along the filter walls, preserves the beneficial effect of the ash membrane from preventing soot depth filtration, and provides some reduction in pressure drop.
Conventional forced air systems accomplish exactly the opposite of the ideal cleaning method proposed above. These systems and methods remove much of the ash in the layer along the channel walls (region of the DPF subject to highest flows), but are ineffective at removing ash in the plugged regions (most of the ash) of the filter which experience no, or negligible, flows.
Therefore, an improved cleaning system and method for use with filters, and particularly diesel particulate filters, is needed.